Fuel cell stack including ejector and blower for anode recirculation and method for controlling the same

ABSTRACT

The present invention relates to a fuel cell system for vehicles and a method for controlling the same which stably maintains an output of a fuel cell by precisely estimating a recirculated hydrogen amount to a stack. A fuel cell system according to the present invention may include: a stack comprising a plurality of unit cells for generating electrical energy by electrochemical reaction of a fuel and an oxidizing agent; a blower for recirculating a gas exhausted from the stack so as to supply the gas back to the stack; an ejector for recirculating the gas exhausted from the stack, receiving hydrogen so as to mix the hydrogen to the recirculated gas, and supplying the mixture to the stack; a sensor module for detecting a driving condition of the vehicle; and a control portion for controlling operations of the blower and the ejector by using the driving condition of the vehicle and performance maps of the blower and the ejector.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application from U.S. patentapplication Ser. No. 12/960,812 filed on Dec. 6, 2010, issued as U.S.Pat. No. 8,709,669 and claims priority to and the benefit of KoreanPatent Application No. 10-2010-0074234 filed in the Korean IntellectualProperty Office on Jul. 30, 2010, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention generally relates to a fuel cell system forvehicles and a method for controlling the same. More particularly, thepresent invention relates to a fuel cell system for vehicles and amethod for controlling the same which stably maintains an output of afuel cell by precisely estimating a recirculated hydrogen amount to afuel cell stack.

(b) Description of the Related Art

A fuel cell system is a well known type of electric generator systemwhich converts chemical energy of a fuel directly into electricalenergy.

The fuel cell system includes a fuel cell stack for generatingelectrical energy, a fuel supply for supplying a fuel (that is,hydrogen) to the fuel cell stack, an air supply for supplying oxygen inair (which is an oxidizing agent for the electrochemical reaction) tothe fuel cell stack, and a device for managing heat and water forradiating reaction heat of the fuel cell stack to an exterior of thesystem and controlling operating temperature of the fuel cell stack.

Therefore, the fuel cell system generates electricity by anelectrochemical reaction of the hydrogen, which is the fuel, and theoxygen in the air, and exhausts heat and water which are by-product ofthe reaction.

The fuel cell stack as applied to a fuel cell vehicle includes aplurality of unit batteries arranged sequentially. Each unit batteryincludes a membrane-electrode assembly (MEA) disposed at the innermostpart thereof, and the membrane-electrode assembly includes anelectrolyte membrane for transferring hydrogen ions, and catalyticlayer. In particular, a cathode and an anode are spread at both sides ofthe electrolyte membrane so as to react the hydrogen with the oxygen. Inaddition, a gas diffusion layer (GDL) is positioned at an exteriorportion of the membrane-electrode assembly (MEA). In particular, a GDLIs positioned at the exterior portion in which the cathode and the anodeare positioned. Further, a separator is positioned at an exterior of thegas diffusion layer. The separator is formed of a flow field forsupplying the fuel and the air to the cathode and the anode and forexhausting water generated by the reaction.

Thus, in a fuel cell, the hydrogen and the oxygen are ionized by achemical reaction at each catalytic layer such that the hydrogenundergoes an oxidation reaction so as to generate hydrogen ions andelectrons, and oxygen ions undergo a reduction reaction with thehydrogen ions so as to generate water. In particular, since the hydrogenis supplied to the anode (which is also referred to as an “oxidationelectrode”) and the oxygen (or air) is supplied to the cathode (which isalso referred to as a “reduction electrode”), the hydrogen supplied tothe anode is ionized into hydrogen ions (H+) and electrons (e−) by thecatalyst of the electrode layer formed at both sides of the electrolytemembrane. After that, only the hydrogen ions selectively pass throughthe electrolyte membrane, which is a cation-exchange membrane, and istransferred to the cathode. Simultaneously, the electrons (e−) aretransferred to the cathode through the gas diffusion layer and theseparator, which are conductors.

Therefore, the hydrogen ions supplied to the cathode through theelectrolyte membrane and the electrons supplied to the cathode by theseparator react with the oxygen in the air supplied to the cathode by anair supply so as to generate water.

At this time, movement of the hydrogen ions causes electrons to flowthrough an exterior conducting wire thereby generating current. When thewater is generated by the reaction, heat is also generated.

In order to apply such a fuel cell system to the vehicle, it isimportant to maintain an output of the stack stably, and for thispurpose the amount of recirculated hydrogen provided to the stack shouldbe precisely detected or estimated.

However, since a gas recirculated in a fuel cell vehicle has a highwater content, and since the gas passes through a short recirculationpassage, it is difficult to directly detect the recirculated hydrogenamount by means of a flowmeter.

Therefore, the recirculated hydrogen amount has been typically estimatedby using a thermal equilibrium equation. According to the thermalequilibrium equation, heat acquired from the supplied hydrogen is thesame as heat lost by the recirculated gas. According to this thermalequilibrium equation, if the detected values (e.g., temperature,pressure, hydrogen concentration, and so on) are precise, a reliablerecirculated hydrogen amount may be calculated.

However, the prediction of recirculated hydrogen amount using thethermal equilibrium equation depends greatly on temperatures. Inparticular, the prediction of the recirculated hydrogen amount using thethermal equilibrium equation is precise when equilibrium temperature isreached. However, the time required to reach equilibrium temperature isvery long in an actual system. Therefore, a method for predicting therecirculated hydrogen amount by using the thermal equilibrium equationcannot be applied to an actual system.

In addition, since various heat generations/losses, such as heatgenerated by a blower, expansion heat of the hydrogen, condensation heatof mixed gas, and heat loss in the line, exist in an actual system,taking these effects into consideration makes the calculation verycomplex.

The above information disclosed in this Background section is only forenhancement of understanding of the background of the invention andtherefore it may contain information that does not form the prior artthat is already known in this country to a person of ordinary skill inthe art.

SUMMARY OF THE INVENTION

The present invention provides a fuel cell system for vehicles and amethod for controlling the same having advantages of maintaining aconstant output of a stack by precisely predicting a recirculatedhydrogen amount and maintaining the recirculated hydrogen amount to belarger than a predetermined hydrogen amount.

A fuel cell system for vehicles according to one embodiment of thepresent invention may include: a stack made up of a plurality of unitcells for generating electrical energy by electrochemical reaction of afuel and an oxidizing agent; a blower for recirculating a gas exhaustedfrom the stack so as to supply the gas back to the stack; an ejector forrecirculating the gas exhausted from the stack, receiving hydrogen so asto mix the hydrogen with the recirculated gas, and supplying the mixtureto the stack; a sensor module for detecting a driving condition of thevehicle; and a control portion for controlling operations of the blowerand the ejector by using the driving condition of the vehicle andperformance maps of the blower and the ejector.

According to various embodiments, a recirculated hydrogen amountaccording to a pressure difference between an inlet and an outlet of thestack and a blower RPM may be stored in the performance map of theblower. Further, a recirculated hydrogen amount according to thepressure difference between the inlet and the outlet of the stack and ahydrogen amount supplied to the ejector may be stored in the performancemap of the ejector.

In various embodiments, the fuel cell system may further include a purgevalve for discharging impurities in the stack. The control portion maybe further configured to calculate hydrogen concentration according tothe driving condition of the vehicle, and to control the opening andclosing of the purge valve according to the calculated hydrogenconcentration.

In various embodiments, the fuel cell system may further include acondensed water discharge valve for discharging condensed water in thestack. The control portion may further be configured to calculate avapor amount according to the driving condition of the vehicle, and tocontrol the opening and closing of the condensed water discharge valveaccording to the calculated vapor amount.

In some embodiments, the sensor module may include: a first temperaturesensor for detecting an inlet temperature of the stack; a secondtemperature sensor for detecting an outlet temperature of the stack; afirst pressure sensor for detecting an inlet pressure of the stack; asecond pressure sensor for detecting an outlet pressure of the stack; anRPM sensor for detecting the blower RPM; and an ammeter for detecting acurrent output from the stack.

According to another embodiment of the present invention, a method isprovided for controlling a fuel cell system for vehicles. In particular,the method may include: detecting a driving condition of the vehicle;calculating a recirculated hydrogen amount by using the drivingcondition of the vehicle and performance maps of the blower and theejector; calculating a stoichiometric ratio (SR) of the hydrogen in thestack; comparing the SR of the hydrogen in the stack with apredetermined value; and increasing the recirculated hydrogen amount ifthe SR of the hydrogen in the stack is smaller than the predeterminedvalue.

In some embodiments, the SR of the hydrogen in the stack may becalculated by dividing a sum of the hydrogen amount directly supplied tothe stack and the recirculated hydrogen amount supplied to the stack bythe hydrogen amount directly supplied to the stack.

The recirculated hydrogen amount may be increased if desired byincreasing the hydrogen amount supplied to the ejector and/or byincreasing the blower RPM.

In some embodiments, the method for controlling a fuel cell system forvehicles may further include: calculating a hydrogen concentrationaccording to the driving condition of the vehicle; comparing thecalculated hydrogen concentration with a predetermined hydrogenconcentration; and opening the purge valve if the calculated hydrogenconcentration is lower than the predetermined hydrogen concentration.

In some embodiments, the hydrogen concentration according to the drivingcondition of the vehicle may be predetermined as a function of a purgecycle and a current output.

In some embodiments, the method for controlling a fuel cell system forvehicles may further include: calculating a vapor amount according tothe driving condition of the vehicle; comparing the calculated vaporamount with a predetermined vapor amount; and opening the condensedwater discharge valve if the calculated vapor amount is larger than thepredetermined vapor amount.

In some embodiments, the vapor amount according to the driving conditionof the vehicle may be predetermined as a function of an inlettemperature of the stack, an outlet temperature of the stack, an inletpressure of the stack, and an outlet pressure of the stack.

It is understood that the term “vehicle” or “vehicular” or other similarterm as used herein is inclusive of motor vehicles in general such aspassenger automobiles including sports utility vehicles (SUV), buses,trucks, various commercial vehicles, watercraft including a variety ofboats and ships, aircraft, and the like, and includes hybrid vehicles,electric vehicles, plug-in hybrid electric vehicles, hydrogen-poweredvehicles and other alternative fuel vehicles (e.g. fuels derived fromresources other than petroleum). As referred to herein, a hybrid vehicleis a vehicle that has two or more sources of power, for example bothgasoline-powered and electric-powered vehicles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a fuel cell system for vehiclesaccording to an exemplary embodiment of the present invention.

FIG. 2 is a block diagram of a fuel cell system for vehicles accordingto an exemplary embodiment of the present invention.

FIG. 3 is a flowchart of a method for controlling a fuel cell system forvehicles according to an exemplary embodiment of the present invention.

FIG. 4 shows one example of a blower performance map used in a methodfor controlling a fuel cell system for vehicles according to anexemplary embodiment of the present invention.

FIG. 5 shows one example of an ejector performance map used in a methodfor controlling a fuel cell system for vehicles according to anexemplary embodiment of the present invention.

FIG. 6 is a graph showing a recirculated hydrogen amount, an anode SR,and a current when a method for controlling a fuel cell system forvehicles according to an exemplary embodiment of the present inventionis used.

DETAILED DESCRIPTION OF THE EMBODIMENTS

An exemplary embodiment of the present invention will hereinafter bedescribed in detail with reference to the accompanying drawings.

A fuel cell system according to an exemplary embodiment of the presentinvention is provided in a fuel cell vehicle and is operated as anelectric generator system which generates electrical energy by anelectrochemical reaction of a fuel and an oxidizing agent.

If the fuel cell system is a direct oxidation fuel cell, the fuel caninclude, for example, alcoholic liquid fuel such as methanol and ethanoland hydrocarbon family liquefied gas fuel, the chief ingredients ofwhich are methane, ethane, propane, and butane.

On the other hand, if the fuel cell system is a polymer electrolytemembrane fuel cell, the fuel can include, for example, reforming gasgenerated from the liquid fuel or liquefied gas fuel by a reformer.

For convenience of explanation, the fuel will be referred to as hydrogenin this specification. Or course, it is understood that the presentinvention is not limited only to hydrogen fuel, and other known fuelsare encompassed by the present invention.

In addition, according to the exemplary embodiment of the invention, theoxidizing agent may be oxygen gas stored in a special tank or may benatural air. However, as described herein, the oxidizing agent will bereferred to as air. Or course, it is understood that the presentinvention is not limited only to air, and other known oxygen agents areencompassed by the present invention

FIG. 1 is a schematic diagram of a fuel cell system for vehiclesaccording to an exemplary embodiment of the present invention. As shownin FIG. 1, the fuel cell system for vehicles according to thisembodiment includes a compressor 10, a hydrogen storage 20, a stack 30,an outlet manifold 40, a blower 50, an ejector 60, and a mixing chamber70.

As shown, the compressor 10 is in connection with the stack 30 throughan air supply line 12. The compressor 10 pressurizes air and suppliesthe air to the stack 30. In addition, in some embodiments, a valve (notshown) for controlling air supply may be provided in the air supply line12.

Hydrogen is stored in the hydrogen storage 20, and the stored hydrogencan be supplied to the ejector 60 and/or the mixing chamber 70. Forexample, the hydrogen storage 20 can be in connection with the mixingchamber 70 through a first hydrogen supply line 22, and can also be inconnection with the ejector 60 through a second hydrogen supply line 24.Valves (not shown) for controlling hydrogen supply may be furtherprovided in the first and second hydrogen supply lines 22 and 24. Insome embodiments, if desired, the hydrogen may be supplied only throughone of the supply lines (e.g. only the second hydrogen supply line 24)by removing the other supply line (e.g. by removing the first hydrogensupply line 22), or for example by closing a valve positioned in one ofthe supply lines.

The stack 30 is made up of a plurality of unit cells 11 sequentiallyarranged. Each unit cell generates electrical energy throughelectrochemical reaction of fuel and air. Each unit cell may be apolymer electrolyte membrane fuel cell or a direct oxidation fuel cellaccording to the fuel used therein.

The unit cell includes a membrane-electrode assembly (MEA)(not shown,and which can be in accordance with any known MEA) and a plurality ofseparators in contact with and disposed on both sides of themembrane-electrode assembly. In some embodiments, the separator is plateshaped and has conductivity, and is provided with a channel for flowingthe fuel and the air to a surface in close contact with themembrane-electrode assembly.

Further, an anode electrode (hereinafter, referred to as an “anode”) isprovided at one surface of the membrane-electrode assembly, and acathode electrode (hereinafter, referred to as a “cathode”) is providedat the other surface of the membrane-electrode assembly. In addition, anelectrolyte membrane is provided between the anode and the cathode.

The anode ionizes the fuel supplied through the channel of the separatorinto electrons and hydrogen ions by an oxidation reaction, and theelectrolyte membrane transports the hydrogen ions to the cathode. Thecathode generates water and heat though a reduction reaction between theelectrons and the hydrogen ions supplied from the anode, and oxygen inthe air supplied through the channel of the separator.

An air exhaust line 14 can further be in connection with the cathode ofthe stack 30 such that the air going through the chemical reaction isexhausted to the exterior of the vehicle through the air exhaust line14.

In addition, nitrogen, which is in the air supplied to the cathode ofthe stack 30, penetrates the electrolyte membrane and passes to theanode, to thereby increase nitrogen concentration in the anode. If thenitrogen concentration in the anode increases to certain levels,diffusion of the hydrogen is hindered. Therefore, it is preferred thatnitrogen in the anode be removed.

Further, a portion of the water generated in the cathode by the chemicalreaction penetrates the electrolyte membrane and passes to the anode. Ifthe water flowing to the anode remains in a catalytic layer, then thecatalyst's reaction capacity is reduced. In addition, if the waterflowing to the anode remains in the channel, the path for supplying thehydrogen becomes blocked. Therefore, water in the catalytic layer andthe channel of the anode should be removed. In some embodiments, theanode of the stack 30 can be connected to a purge line 36, and a purgevalve 80 or the like can be mounted at the purge line 36. The purgevalve 80 can be configured to open at certain times, such as everypredetermined purge cycle, or as necessary so as to exhaust impurities(i.e., nitrogen and water) in the anode.

In some embodiments, the anode of the stack 30 can be connected to acondensed water exhaust line 38, and a condensed water discharge valve82 or the like can be mounted at the condensed water exhaust line 38. Ifthe condensed water discharge valve 82 is opened, the water in thecatalytic layer and/or the channel of the anode is exhausted. In someembodiments, instead of using an additional condensed water dischargevalve 82, the purge valve 80 can also be used as the condensed waterdischarge valve 82.

As further shown in FIG. 1, the stack 30 is in connection with arecirculation inlet line 32 so as to receive a recirculated gas, and isin connection with a recirculation outlet line 34 so as to exhaust a gasgenerated by the reaction of the air and the hydrogen in the stack 30.

As shown in FIG. 1, an outlet manifold 40 can be in connection with thestack 30 through a recirculation outlet line 34. Therefore, gasgenerated at the stack 30 can be gathered in the outlet manifold 40 andrecirculated through two paths. In particular, the outlet manifold 40can be in connection with the blower 50 through a first recirculationline 42, and can be in connection with the ejector 60 through a secondrecirculation line 44. The outlet manifold 40 is not particularlylimited and can be in accordance with any convention design, and isgenerally configured so as to provide a location at which the gasgenerated at the stack 30 can gather.

The blower 50, as shown in FIG. 1, can be in connection with the outletmanifold 40 through the first recirculation line 42, and can be inconnection with the mixing chamber 70 through a first connecting line46. As such, the blower 50 can recirculate gas gathered at the outletmanifold 40 and supply the gas to the mixing chamber 70.

As further shown, the ejector 60 is in connection with the outletmanifold 40 through the second recirculation line 44, and is inconnection with the mixing chamber 70 through a second connecting line48. In addition, the ejector 60 can be in connection with the hydrogenstorage 20 through the second hydrogen supply line 24. As such, theejector 60 can recirculate gas gathered at the outlet manifold 40, mixhydrogen with the recirculated gas, and supply the mixture to the mixingchamber 70.

As further shown in FIG. 1, the mixing chamber 70 is in connection withthe hydrogen storage 20 through the first hydrogen supply line 22, is inconnection with the blower 50 through the first connecting line 46, isin connection with the ejector 60 through the second connecting line 48,and is in connection with the stack 30 through the recirculation inletline 32. As such, the mixing chamber 70 can mix the hydrogen suppliedfrom the hydrogen storage 20 with the gas recirculated through theblower 50 and the ejector 60, and supply the mixture to the stack 30.The mixing chamber 70 is not particularly limited, and can be inaccordance with any conventional mixing chamber, and is generallyconfigured so as to provide a place at which recirculated gas andhydrogen can be mixed.

As shown, according to the exemplary embodiment of the present inventionshown in FIG. 2, the fuel cell system for vehicles further includes asensor module and a control portion 170.

The sensor module is configured to detect a driving condition of thevehicle, and can include, for example, first and second temperaturesensors 110 and 120, first and second pressure sensors 130 and 140, anRPM sensor 150, and an ammeter 160. In addition, a plurality of sensorsfor detecting the driving condition of the vehicle may be included inthe sensor module. However, in the following description, the sensorsused in a method for controlling a fuel cell system for vehiclesaccording to an exemplary embodiment of the present invention will bedescribed in further detail.

According to this exemplary embodiment, the first temperature sensor 110is mounted at the inlet of the stack 30 or the recirculation inlet line32, and is configured to detect an inlet temperature of the stack 30,and to transmit a signal corresponding thereto to the control portion170. The second temperature sensor 120 is mounted at the outlet of thestack 30 or the recirculation outlet line 34, and is configured todetect an outlet temperature of the stack 30, and to transmit a signalcorresponding thereto to the control portion 170.

Further, according to this exemplary embodiment, the first pressuresensor 130 is mounted at the inlet of the stack 30 or the recirculationinlet line 32, and is configured to detect an inlet pressure of thestack 30, and to transmit a signal corresponding thereto to the controlportion 170. The second pressure sensor 140 is mounted at the outlet ofthe stack 30 or the recirculation outlet line 34, and is configured todetect an outlet pressure of the stack 30, and to transmit a signalcorresponding thereto to the control portion 170.

Further, the RPM sensor 150 is configured to detect a rotation speed ofthe blower 50 (blower RPM), and to transmit a signal correspondingthereto to the control portion 170.

The ammeter 160 is configured to detect the current generated in thestack 30, and to transmit a signal corresponding thereto to the controlportion 170. It is noted that the current generated in the stack 30 isalmost proportional to an output of the stack 30, and may be a basis forpredicting the output of the stack 30 or a load of the vehicle.

As shown, the control portion 170 receives signals corresponding to thedriving condition of the vehicle from the sensor modules and controlsall of the valves (e.g., condensed water discharge valve 82, purge valve80, and so on), the ejector 60, and the blower 50. As such, the controlportion 170 is electrically in connection with to the sensor module, thevalves, the blower 50, and the ejector 60.

Hereinafter, a method for controlling the fuel cell system for vehiclesaccording to the exemplary embodiment of the present invention will befurther described in detail in connection with FIG. 3 which is aflowchart of the exemplary method.

As shown in FIG. 3, the method for controlling the fuel cell system forvehicles begins by detecting the driving condition of the vehicle atstep S200. The driving condition of the vehicle includes, for example,the inlet and outlet temperatures of the stack 30, the inlet and outletpressures of the stack 30, the rotation speed of the blower 50, and thecurrent output of the stack 30.

The control portion 170 then calculates a vapor amount according to thedriving condition of the vehicle at step S210. The vapor amount W[%]according to the driving condition of the vehicle can be calculated fromEquation 1.W[%]=f(T1,T2,P1,P2)  (Equation 1)

Herein, T1 denotes the inlet temperature of the stack 30, T2 denotes theoutlet temperature of the stack 30, P1 denotes the inlet pressure of thestack 30, and P2 denotes the outlet pressure of the stack 30. As such,if the inlet and outlet temperatures and the inlet and outlet pressuresof the stack 30 are known, the vapor amount can be provided by the idealgas equation or by experimental values.

After the vapor amount is calculated, the control portion 170 determineswhether the calculated vapor amount is smaller than or equal to apredetermined vapor amount at step S220. The predetermined vapor amount,for example, may be 15% or any other suitable amount which can bedetermined by one of skill in the art.

If the vapor amount in the stack 30 is higher than the predeterminedvapor amount, then it is difficult for the oxidation-reduction reactionin the stack 30 to occur. Therefore, if the calculated vapor amount ishigher than the predetermined vapor amount, then the control portion 170opens the condensed water discharge valve 82 so as to discharge thecondensed water in the stack 30 at step S280, thereafter, the controlportion 170 returns to step S210.

At this time, if the vapor amount in the stack 30 is smaller than orequal to the predetermined vapor amount at step S220, then the controlportion 170 calculates the hydrogen concentration according to thedriving conditions of the vehicle at step S230. The hydrogenconcentration H[%] according to the driving condition of the vehicle canbe calculated from Equation 2.H[%]=f(Tpurge,ΣC)  (Equation 2)

wherein, Tpurge denotes the purge cycle, and C denotes the currentoutput of the stack 30.

As described above, if the nitrogen moves from the cathode to the anode,the hydrogen concentration is reduced. In addition, if the purge valve80 opened, the nitrogen concentration is reduced and the hydrogenconcentration increases. Further, the nitrogen concentration of theanode relates to the hydrogen used in the stack 30. As such, thehydrogen concentration in the stack 30 is related to (as shown inEquation 2) the purge cycle and the current output of the stack 30.Thus, Equation 2 may be obtained by experimentation.

If the hydrogen concentration is calculated at step S230, the controlportion 170 determines whether the hydrogen concentration in the stack30 is higher than or equal to a predetermined hydrogen concentration ata step S240. The predetermined hydrogen concentration may be 80%, or anyother suitable amount which can be determined by one of skill in theart.

If the hydrogen concentration in the stack 30 is lower than thepredetermined hydrogen concentration, the nitrogen concentration in thestack 30 is high and it is difficult for the oxidation-reductionreaction to occur. In this case, the control portion 170 can opens thepurge valve 80 so as to exhaust the impurities (e.g., nitrogen andwater) in the stack 30, and then return to steps S290 and S230.

If the hydrogen concentration in the stack 30 is higher than or equal tothe predetermined hydrogen concentration at step S240, then the controlportion 170 can calculate the recirculated hydrogen amount by using theperformance maps of the blower 50 and the ejector 60 at step S250. StepS250 will be described in further detail in connection with FIG. 4,which shows the recirculated hydrogen amount according to the rotationspeed of the blower 50 and a pressure difference between the inlet andthe outlet of the stack 30, which is determined by running experimentsand gathering data so as to generate a blower performance map. Forexample, FIG. 4 shows one example of a blower performance map. Ofcourse, this is only one example, and the blower performance map is notlimited to the one shown in FIG. 4. In particular, the blowerperformance map may be produced according to the inlet and outlettemperatures of the stack 30, a composition ratio of the gas (i.e.,composition ratios of hydrogen/nitrogen/vapor) as well as the rotationspeed of the blower 50 and the pressure difference between the inlet andthe outlet of the stack 30.

If the blower performance map is produced, the recirculated hydrogenamount by the blower 50 is functionalized as Equation 3.Q[rec,H2,HRB]=f{H[%],T2,P1,P2,RPM}  (Equation 3)

wherein, Q[rec, H2, HRB] denotes the recirculated hydrogen amount by theblower 50, and an RPM denotes the rotation speed of the blower 50.

If the driving condition of the vehicle detected at step S200 is inputinto Equation 3, then the recirculated hydrogen amount by the blower 50can be calculated.

Similarly, the recirculated hydrogen amount by the ejector 60 can becalculated as follows.

As shown in FIG. 5, the recirculated hydrogen amount according to thehydrogen amount supplied to the ejector 60 and the pressure differencebetween the inlet and the outlet of the stack 30 is detected byperforming a number of experiments, gathering the experimental data, andproducing an ejector performance map. FIG. 5 shows one example of anejector performance map. Of course, this is only one example, and theejector performance map is not limited to the one shown in FIG. 5. Theejector performance map may be produced according to the inlet andoutlet temperatures of the stack 30, a composition ratio of the gas(i.e., composition ratios of hydrogen/nitrogen/vapor) as well as thehydrogen amount supplied to the ejector 60 and the pressure differencebetween the inlet and the outlet of the stack 30.

If the ejector performance map is produced, the recirculated hydrogenamount by the ejector 60 is functionalized as Equation 4.Q[rec,H2,Ejector]=f{Hs,H[%],T2,P1,P2}  (Equation 4)

wherein, Q[rec, H2, Ejector] denotes the recirculated hydrogen amount bythe ejector 60, and Hs denotes the hydrogen amount supplied to theejector 60.

If the driving condition of the vehicle detected at step S200 is inputinto Equation 4, the recirculated hydrogen amount by the ejector 60 canbe calculated.

If the recirculated hydrogen amount by the blower 50 and the ejector 60is calculated, the control portion 170 can then calculate an SR(Stoichiometric Ratio) of the hydrogen in the stack 30 (particularly,anode) at step S260. Hereinafter, the SR of the hydrogen in the anodewill be referred to as an anode SR. The anode SR can be calculated fromEquation 5 and Equation 6.Q[rec,H2]=Q[rec,H2,HRB]+Q[rec,H2,Ejector]  (Equation 5)anode SR={Q[H2,Supply]+Q[rec,H2]}/Q[H2,Supply]  (Equation 6)

wherein, Q[rec, H2] denotes a total hydrogen amount recirculated to thestack 30, and Q[H2, Supply] denotes the hydrogen amount supplied fromthe hydrogen storage 20.

If the anode SR is calculated, the control portion 170 can determineswhether the anode SR is bigger than or equal to a predetermined value atstep S270. The predetermined value may be 1.5, or any other suitablevalue which can be determined by one of skill in the art.

If the anode SR is smaller than the predetermined value, then thecontrol portion 170 controls the blower 50 and the ejector 60 toincrease the recirculated hydrogen amount to the stack 30 because therecirculated hydrogen amount to the stack 30 is small. In particular,the control portion 170 can increase the rotation speed of the blower 50at step S310 and increase the hydrogen amount supplied to the ejector 60at step S300. The control portion 170 may perform both step S300 andstep S310, or may perform any one of step S300 and step S310.

If the anode SR is bigger than or equal to the predetermined value atstep S270, then the control portion 170 repeats the method forcontrolling the fuel cell system for vehicles according to the exemplaryembodiment of the present invention from the beginning.

It is noted that the present method is not limited to the exemplaryembodiment of the present invention, for example, in the order asdescribed above.

FIG. 6 is a graph showing a recirculated hydrogen amount, an anode SR,and a current in a case that a method for controlling a fuel cell systemfor vehicles according to an exemplary embodiment of the presentinvention is used.

As shown in FIG. 6, the recirculated hydrogen amount is reduced if aload (current) of the vehicle is reduced, and the recirculated hydrogenamount increases if the load (current) of the vehicle increases. Thisshows that the recirculated hydrogen amount is suitably managedaccording to a load change of the vehicle.

As described above, pressure values having quick responsiveness can beused for predicting a recirculated hydrogen amount, and, thus,prediction of the recirculated hydrogen amount has high reliabilityaccording to the present invention.

Further, according to the present invention, since a constant amount ofhydrogen can be reliably recirculated by precisely predicting therecirculated hydrogen amount, output of a stack may be maintained to beconstant.

Further, since a blower, an ejector, a condensed water discharge valve,and a purge valve can be suitably controlled so as to increase therecirculated hydrogen amount, performance of a fuel cell system may beimproved in accordance with the present invention.

While this invention has been described in connection with what ispresently considered to be practical exemplary embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. A fuel cell system for vehicles comprising: astack comprising a plurality of unit cells for generating electricalenergy by electrochemical reaction of a fuel and an oxidizing agent; ablower configured to recirculate a gas exhausted from the stack so as tosupply the gas back to the stack; an ejector configured to recirculatethe gas exhausted from the stack, receive hydrogen so as to mix thehydrogen with the gas recirculated, and supply a mixture of the hydrogenand the gas to the stack; a sensor module configured to detect a drivingcondition of the vehicle; and a control portion configured to controloperations of the blower and the ejector based on the driving conditionof the vehicle and performance maps of the blower and the ejector,wherein a first recirculated hydrogen amount according to a pressuredifference between an inlet and an outlet of the stack and a blower RPMis stored in the performance map of the blower, and a secondrecirculated hydrogen amount according to the pressure differencebetween the inlet and the outlet of the stack and a hydrogen amountsupplied to the ejector is stored in the performance map of the ejector.2. The fuel cell system of claim 1, further comprising a purge valveconfigured to discharge impurities in the stack and to open and closebased on a hydrogen concentration calculated according to the drivingcondition of the vehicle.
 3. The fuel cell system of claim 1, furthercomprising a condensed water discharge valve configured to dischargecondensed water in the stack and to open and close based on a vaporamount calculated according to the driving condition of the vehicle. 4.The fuel cell system of claim 1, wherein the sensor module comprises: afirst temperature sensor configured to detect an inlet temperature ofthe stack; a second temperature sensor configured to detect an outlettemperature of the stack; a first pressure sensor configured to detectan inlet pressure of the stack; a second pressure sensor configured todetect an outlet pressure of the stack; an RPM sensor configured todetect the blower RPM; and an ammeter configured to detect a currentoutput from the stack.